Aircraft systems and pressure sensors installed on them can be subjected to high voltage inputs from lightning strikes that can damage unprotected sensors. Since sensor housings are typically metal (stainless steel) and installed on other metal assemblies (such as a pump or manifold), the entire system may be conductive and susceptible to these high voltage inputs. Therefore, the sensors need to protected and isolated or electrically insulated from adjacent metal components. Typical thin film pressure sensors are rated to withstand 500 VAC of power.
However many aircraft systems require sensors to survive 1500 VAC of power spikes. Moreover, aircraft pressure sensors are routinely subjected to harsh environmental conditions. Since these sensors often measure hydraulic pressure, the sensors can be exposed to the caustic nature of some hydraulic oils. Also, pressures often reach 4500 psi and temperatures can range between −55° C. to 200° C., causing sensor components to be subjected to substantial compressive forces, wear, creep and distortion. The sensors may also be subject to impulse pressure cycling, which can result in sensor failure due to fatigue.
Two types of pressure sensors are appropriate for use on aircraft: MEMS (micro-electromechanical) and Thin Film sensors. Both types use piezoresistive strain gauges to convert pressure into an electrical output. The most common is a MEMS sensor. The strain gauge for this sensor is molecularly bonded on to a silicone substrate and is rated to 1500 VAC. In this case, the silicone acts as an insulating material for the strain gauge. However, due to its construction, this type of sensor is more expensive than a thin film sensor and cannot come in contact with harsh system medias being measured, like fuel or hydraulic oils. Therefore, it is housed in a sealed metal pressure capsule filled with silicone oil and covered by a thin stainless diaphragm. In this case, the media pressure is applied to the stainless diaphragm and transferred by the silicone oil to the MEMS sensor. A second type of sensor is a metal thin film sensor. This sensor has a metal housing and metal diaphragm on which the strain gauge sensor is directly deposited. One of the main benefits of this construction is that the measured media can be applied directly to the diaphragm under the strain gauge. However, due to its metal housing and diaphragm, this type of sensor will not withstand 1500 VAC and has a lower dielectric rating of approximately 500 VAC.
A metal thin film sensor uses changes in resistance in a Wheatstone bridge strain gauge structure, due to the metal diaphragm deflection, to measure pressure. As noted, the base 14 of the sensor body is made of stainless steel. The resistance structure is produced by photolithography. Thin film measuring cells stand out due to their excellent resistance to pressure peaks and bursting pressure. Extremely high pressures can be measured—even when exposed to high shock and vibration loads. In metal thin film technology, four resistors are interconnected to create the Wheatstone bridge.